Almost all metallic materials used in industries are extracted from raw ores and restored into their original metallic states in time. Thus, as time passes, metallic materials used to construct industrial structures or buildings react with an environment and are inevitably corroded or oxidized. This corrosion is mainly generated by electrochemical reactions caused by the movement of electrons, and thus it is called electrochemical corrosion. Metallic structures are in corrosive battery states while corrosion is progressing such that a corrosive potential is generated and a predetermined corrosive current flows through the metallic structures.
In general, corrosion protection is to eliminate or suppress one or more conditions from main causes of corrosion. Electric protection is a method for suppressing corrosion of facilities or structures mainly by artificially adjusting a potential or current of facilities or structures requiring such protection. Electric protection includes anodic protection of anodizing an object to be corrosion-protected and cathodic protection of cathodizing the object to be corrosion-protected. In the case of anodic protection, when potential adjustment is not precisely performed, corrosion may be accelerated. Thus, anodic protection is used under certain conditions, and cathodic protection is usually used.
Cathodic protection is a method for preventing corrosion by artificially reducing the potential of an object to be corrosion-protected. The cathodic protection is divided into a sacrificial anodic method and an external power method by the manner of applying an anticorrosive current. In the sacrificial anodic method, metals that can be easily ionized are electrically connected in an electrolyte to act as an anode, thereby cathodizing the object to be corrosion-protected. In the external power method, a cathode (−) of a DC power supply or a rectifier is connected to the object to be corrosion-protected, and an anode (+) of the DC power supply or the rectifier is connected to a cathode member, thereby obtaining an anticorrosive current.
Meanwhile, exhaust gases generated from combustion facilities such as a steam power plant and an incinerator, or sulfuric compounds contained in a hot and humid gas generated in a general chemical plant are changed into a sulfurous acid gas by the following reaction.SO2+O2→SO3+O
The sulfurous acid gas reacts with water below a dew point and is balanced as H2—SO3—H2SO4. In addition, the sulfurous acid gas is condensed on a metallic surface below a dew point comparatively lower than that of a flowing gas generated in bottoms, walls, or ceilings of exhaust gas-family facilities and exists in the form of a thick film and thin film high-concentration sulfuric solution. The sulfuric solution causes serious corrosion of materials such as high alloy steel and coating, on the basis of the period of starting and suspension of facilities. In addition, the sulfuric solution causes a harmful gas to leak out of corroded and damaged facilities and serious environmental problems. Thus, desulfurization facilities are made of high-priced special anticorrosive alloy in consideration of corrosion. However, the desulfurization facilities are easily corroded in a hot and humid environment which is a characteristic of condensed sulfuric acid and exhaust gas facilities. Due to frequent stoppage of facilities for maintenance, an economical efficiency in operation of facilities is lowered, and for maintenance, high-priced special anticorrosive alloy or lining materials should be repeatedly used, yielding many additional costs. Thus, if the aforementioned electric protection is performed in desulfurization facilities, corrosion of desulfurization facilities can be prevented. As a result, maintenance costs can be reduced, and inferior materials instead of high-priced special anticorrosive steel, can be used in desulfurization facilities. Thus, construction costs can also be reduced.
However, a conventional apparatus and method for electric protection can be used in various fields such as buried piping, ocean facilities, shipping, and cooling systems in a power plant. However, the conventional apparatus and method for electric protection can be used only in an environment where a sacrificial anodic member or an insoluble anodic member is completely dipped in corrosive fluids. Thus, the conventional apparatus and method for electric protection cannot be used in an environment where thin film corrosive fluids are formed, like in a duct of desulfurization facilities. In addition, in the conventional apparatus and method for electric protection, due to high electric conductivity like the case where corrosive fluids are a sulfuric solution which is a waste solution flowing through a duct of desulfurization facilities, if electric protection is performed, the amount of consumption of an anticorrosive current increases. In addition, when an object to be corrosion-protected is used in an environment that severely varies in time and in addition the position of the object in the environment is not constant, it is not easy to determine an anticorrosive current or a potential. As such, it is difficult to use the conventional apparatus and method for the electric protection of metallic structures such as a duct of desulfurization facilities.